Multilocus sequence data reveal extensive departures from equilibrium in domesticated tomato (Solanum lycopersicum L.).

ABSTRACT

Limited genetic variation has been observed within tomato (Solanum lycopersicum L.), although no studies have extensively surveyed single nucleotide polymorphism (SNP) diversity among tomato landraces. We estimated intraspecific DNA sequence variation by analyzing 50 gene fragments (23.2 kb) per plant in a 31 plant diversity panel. The majority of loci (80%) were polymorphic with the minor allele at a frequency of 10% or less for most (141 of 155) SNPs. Mean diversity as estimated by theta and pi was approximately 1.5 SNPs per kb. Significant linkage disequilibrium was observed between 19% of locus pairs, and within-locus population recombination estimates were negligible. We also sequenced 43 gene fragments from wild tomato Solanum arcanum Peralta as an outgroup. Various statistical tests rejected a neutral equilibrium model of molecular evolution at 10 of 50 loci. Rare, highly diverged alleles were observed, involving at least seven tomato lines and five loci. Some of these may represent introgressions that originated both from natural hybridization with Solanum pimpinellifolium L. and from crosses with S. pimpinellifolium and additional wild relatives for crop improvement. The former was reported from classical field studies carried out by CM Rick; the latter has been extensively documented in the crop, particularly for transfer of disease resistance alleles. Extensive introgression and frequent bottlenecks within S. lycopersicum will pose a challenge to reconstructing the genetic bases of domestication and selection using methods that rely on patterns of molecular polymorphism.